1. Field of the Invention
The present invention relates to a thermal analog fire detector which detects the temperature of the atmosphere by means of a temperature-sensitive resistance element having a variable resistance according to the temperature and which sends the detected temperature information to a receiver.
2. Description of the Related Art
A thermistor is known as a temperature-sensitive resistance element of this type. Such a thermistor is mainly classified into the following two types. One of them is an NTC thermistor in which electrical resistance decreases exponentially as the temperature increases, as shown in FIG. 8. The other one is a PTC thermistor having a positive temperature coefficient. The NTC thermistor is generally used.
When such a thermistor is used for a thermal analog fire detector, the thermistor (variable resistance TH) and the fixed resistor R are connected in series to each other, and the reference voltage E is divided, as illustrated in FIG. 9. The voltage across the fixed resistor R obtained by the following formula is detected: EQU V{=R.multidot.E/(R+TH)}
Thus, the temperature of the atmosphere can be detected.
As is understood from the foregoing description, in a conventional thermal analog fire detector, the temperature of the atmosphere is measured by the following operation: a thermistor in which electrical resistance varies exponentially relative to the temperature and the fixed resistor R are connected in series to each other, and the reference voltage E is divided. Thus, as shown in FIG. 10, when the fixed resistor R having a greater resistance is selected, the voltage has a small rate of change in a high temperature range, thus lowering measurement precision and resolution. On the other hand, when the resistor R having a smaller resistance is selected, the voltage has a small rate of change in a low temperature range, and measurement precision and resolution are also lowered. As described above, a conventional thermal analog fire detector disadvantageously has low measurement precision and resolution both in high and low temperature ranges. This is particularly problematic for a fire detector because it is required to measure the voltage in a high temperature range with high precision.
In order to overcome such a drawback, the fixed resistor R having an intermediate resistance may be selected and the linear temperature region may be utilized. However, this narrows the range of measuring temperatures which generally falls between -20.degree. C.-100.degree. C. Thus, the fixed resistor having an intermediate resistance cannot cope with such a range.
Also, since the temperature is measured by the with the divided voltages as stated above, the current consumption increases at a high temperature. Thus, the durability of the parts of the peripheral circuit must be taken into account.
Further, a power source is generally supplied to a fire detector from a receiver. Hence, the receiver is usually provided with a battery in case of power failure. Accordingly, a battery having a great capacity is required, and thus a device becomes expensive and the number of detectors which can be connected to the receiver is restricted.
A system for measuring temperatures using such as a thermistor disclosed in U.S. Pat. No. 4,322,725 is known. Such a publication also discloses that the thermistor may be applicable to a fire detecting system. However, according to the thermistor, a signal from the thermistor is also divided into AC and DC portions, thereby obtaining a signal in proportion to the temperature of the heat source. Hence, the foregoing problems particular to the conventional thermistor cannot be solved.